The objective is to obtain a better understanding of human visual processing, especially visual adaptation. The proposed research follows two main directions. The first is to examine and model the effects of adaptation on absolute threshold, increment threshold and apparent brightness in the rod and cone systems. It is argued that recent research of mine and others has shown that there are at least three important adaptation mechanisms: photopigment depletion, a long-term mechanism and a short-term mechanism. The proposed research is deisnged (1) to isolate some of the these mechanisms, and then parametrically measure their effects on absolute threshold, increment-threshold and apparent brightness, and (2) to test several important hypotheses about how these mechanisms work. The second main direction of research is to study and to develop quantitative models of intensity discrimination in the initial image and the short-term afterimage. Recent research of mine has shown that photopic increment thresholds obtained against flashed backgrounds are mediated by the initial image at low background intensities and by a short-term afterimage at high background intensities. The proposed research will (1) test a new model (presented below) of initial-image and afterimage discrimination, (2) measure several new aspects of the short-term cone afterimage, and (3) extend this work to the rod system.